There were no art galleries in North Wales when I was growing up – but there was something better. My first experience of great and awe-inspiring works of art was martial. My paintings were battlements, my sculptures towers.

The castles built by Edward I to rule the Welsh did not strike me as imperial enemies planted in the landscape but as places of imagination and romance. Caernarfon Castle with its polyhedral towers beside the slumbering Straits of Menai was self-evidently a colossus of beauty, an architectural masterpiece whose mathematics of straight lines and sharp angles endures its ruin and mirrors the power of the Snowdonian mountains.

Rhuddlan, more sadly wrecked by Civil War cannon, still has a dignified might as it looks down on its river and across the wide plain towards misty mountains. Best of all, though, and my favourite, was Conwy, whose spiral staircases up and down mysterious towers, wide courtyards where you can play at Robin Hood, and best of all its setting on a craggy outcrop above a roiling rivermouth made it as alluring to me as to JMW Turner.

It's one thing to praise British cathedrals – but if you live in Wales this military medieval heritage is more local, and it is just as exciting.

Some of the greatest artists and architects have designed fortifications: their genius became part of the story of castles. When you visit a church you are hushed, but in a castle you hear the roar of angry voices and clash of arms. A child is more likely to be inspired by a castle than a cathedral. I was. And perhaps more strangely, less familiarly, castles are rule-breaking, inventive, precocious structures that anticipate modernism in surprising, daunting ways.

The dreamy chateaux of France are after all not what most fortresses looked like. Those of Wales were functional as well as aesthetic. The way Conwy arises from its rock, the way Caernarfon's clipped geometries rebuff assault, these features were designed for functional reasons but possess a savage beauty. Is Caernarfon gothic? Is Conwy? This arty question seems irrelevant in the face of their sublime aggressive strength. In castles, there are no rules, and no limits to fantasy.